Topological defect driven thickness changes in layers of bacteria*

Myxococcus xanthus is a rod shaped soil bacterium that lives in collectives of many
millions of cells. These groups of cells exhibit different collective behaviors
which benefit the bacteria depending on the external environment. When M. xanthus are in favorable
conditions, colonies will swarm and spread out to explore new territory, resulting in thin
layers only a few cells thick. Our goal is to understand how the motility
of individual bacteria along with local interactions can lead to the collective behaviors
that exist on length scales significantly larger that of the individuals. Using a
confocal laser scanning microscope we image layers of cells with single cell
resolution while simultaneously obtaining height maps of the layer thickness across the
sample. From these images we are able to extract flow and director fields of the bacteria
over time. Topological defects in director fields are connected to pressure fluctuations
in active nematic systems which can lead to local changes in layer thickness. By combining
height maps with director and flow fields, we are able to examine how the active nematic
nature of the system can result in thickness changes in bacteria layers.